Liquefied petroleum gas (LPG) is a liquefied petroleum-based or natural-gas-based hydrocarbon which is gaseous at an ambient temperature under an atmospheric pressure by compression while optionally cooling, and the main component of it is propane or butane. LPG is advantageously transportable because it can be stored or transported in a liquid form. Thus, in contrast with a natural gas that requires a pipeline for supply, it has a characteristic that it can be filled in a container to be supplied to any place. For that reason, LPG comprising propane as a main component, i.e., propane gas has been widely used as a fuel for household and business use. At present, propane gas is supplied to about 25 million households (more than 50% of the total households) in Japan. In addition to household and business use, LPG is used as a fuel for a portable product such as a portable gas burner and a disposable lighter (mainly, butane gas), an industrial fuel and an automobile fuel.
Conventionally, LPG has been produced by 1) collection from a wet natural gas, 2) collection from a stabilization (vapor-pressure regulating) process of crude petroleum, 3) separation and extraction of a product in, for example, a petroleum refining process, or the like.
LPG, in particular propane gas used as a household/business fuel, can be expected to be in great demand in the future. Thus, it may be very useful to establish an industrially practicable and new process for producing LPG.
As a process for producing LPG, Patent document 1 discloses that a synthesis gas consisting of hydrogen and carbon monoxide is reacted in the presence of a mixed catalyst obtained by physically mixing a methanol synthesis catalyst such as a Cu—Zn-based catalyst, a Cr—Zn-based catalyst and a Pd-based catalyst, specifically a CuO—ZnO—Al2O3 catalyst or a Pd/SiO2 catalyst with a methanol conversion catalyst composed of a zeolite having an average pore size of about 10 Å (1 nm) or more, specifically a Y-type zeolite, to give a liquefied petroleum gas or a mixture of hydrocarbons similar in composition to LPG.
Furthermore, as a process for producing LPG, Non-patent document 1 discloses that, using a hybrid catalyst consisting of a methanol synthesis catalyst such as a 4 wt % Pd/SiO2, a Cu—Zn—Al mixed oxide {Cu:Zn:Al=40:23:37 (atomic ratio)} or a Cu-based low-pressure methanol synthesis catalyst (Trade name: BASF S3-85) and a high-silica Y-type zeolite with SiO2/Al2O3=7.6 treated with steam at 450° C. for 1 hour, C2 to C4 paraffins can be produced in a selectivity of 69 to 85% via methanol and dimethyl ether from a synthesis gas.
On the other hand, Non-patent document 2 discloses a process for producing LPG, using at least one selected from the group consisting of methanol and dimethyl ether as a starting material. Specifically, a starting gas, whose composition is methanol:H2:N2=1:1:1, was passed through the two-layered catalyst layer consisting of ZSM-5 as the former layer and Pt—C as the latter layer (ZSM-5/Pt—C Series) or a mixed catalyst layer consisting of ZSM-5 and Pt—C (ZSM-5/Pt—C Pellet-mixture), under a slightly increased pressure, at a reaction temperature of 603 K (330° C.) and at a methanol-based LHSV of 20 h−1, to carry out an LPG production reaction.